Collision detection in an MMO is just one of those things you may take for granted. If it works, you don't really notice it. If it doesn't work, it's a fun little exploit to hide in a rock and amaze your friends! But collision detection is becoming increasingly important in upcoming MMOs such as Age of Conan and Warhammer Online because of their predominant PvP natures.
So in a recent post at Relmstein's blog, he dissects the different types of collision detection, how they work and in which situations they would be most useful. These situations can range anywhere from player-to-player, player-to-fixed-object or player-to-moving-object collision. Each one requires its own type of algorithm which reacts differently when applied. Most of these detection methods would be set in place to avoid exploit, but can be tricky when it comes to things like line-of-sight.